A federal air marshal on a New York bound flight reportedly left her loaded gun in the cabin toilet where it was discovered by a passenger.
The US agent was flying from Manchester, UK, to John F Kennedy airport on a Delta flight when the incident occurred.
The weapon was handed in to a crew member who returned it to the agent, however she failed to report the slip up to her superiors until several days later, according to the New York Times.
Despite the error, the agent was reportedly placed on another flight just days later.
Air marshal sources told CNN the agent was hired recently and should have been put on leave after the incident.
"She made a mistake because she wasn't given the appropriate tools to succeed," one former air marshal said, pointing to a lack of on-the-job training.
Air Marshal Association president John Casaretti said the Transportation Security Administration (TSA) must improve on training for new hires.
"These rare incidents must be thoroughly investigated and local managers should take appropriate corrective action. A field training officer program and thorough mentorship of new officers can reduce similar performance issues,” he said.
"Air marshals work in punishing conditions, labour under poor leadership and have seen their law enforcement functions curtailed by an administration that lacks vision. The problem is not the air marshals, it's the TSA.”
Both the TSA and Delta were contacted by The Independent for comment.